Conventionally, two-stage charging system weighing apparatuses have been used for weighing powder or granular materials to improve weighing speed and increase weighing accuracy. FIG. 12 represents the relationship of charging weight and charging flow rate with charging time in a two-stage charging system. As shown in FIG. 12, in the two-stage charging system weighing apparatus, a powdery or granular material is supplied to a weighing hopper at a large flow rate until the measurement of the weight of the powdery or granular material reaches about 90% of a target weight (100%) (large quantity charging), and subsequently the remaining 10% of the powdery or granular material is supplied to the weighing hopper at a small flow rate (small quantity charging), in order to increase the weighing speed.
With such a weighing apparatus of a two-stage charging mode, conceivable methods for further increasing the weighing speed while maintaining its weighing accuracy are as follows: (1) the charging weight in the large quantity charging is further increased, for example, to 97% from about 90% as described above; or (2) the quantity of flow of a material to be weighed is increased in the large quantity charging. However, in the foregoing method (1), in which the charging weight in the large quantity charging is increased, overshoot occurs when the flow rate changes at the switchover from the large quantity charging to the small quantity charging, as shown in FIG. 13, resulting in a situation in which the target weight is assumed to be reached and the small quantity charging is not carried out. In this case, the weighed value obtained finally will be less than the target weight. Likewise, in the foregoing method (2), in which the flow rate in the large quantity charging is increased, overshoot also occurs when the flow rate changes at the switchover from the large quantity charging to the small quantity charging, resulting in a situation in which the weighed values obtained finally will be less than the target weight. Further, there is a drawback in that a control for the switchover from the large quantity charging to the small quantity charging is necessary, which increases the cost of the weighing apparatus. In addition, in order to ensure weighing accuracy, it is essential to carry out the small quantity charging, which is time-consuming, and the weighing speed cannot be increased beyond a certain level, which is another problem.
In order to resolve the problems in the weighing apparatus with a two-stage charging mode, a stageless system weighing apparatus has been developed. FIG. 14 represents the relationship of charging weight and charging flow rate with charging time in the stageless system. In the stageless system, its charging flow rate is adjusted so as to decrease continuously from the large quantity charging to the small quantity charging according to a predetermined function so that overshoot does not occur at the switchover from the large quantity charging to the small quantity charging. However, such a stageless system weighing apparatus has a drawback in that a control for the stageless shift from the large quantity charging to the small quantity charging is difficult and, for this reason, cost of the weighing apparatus increases. Also as in the foregoing case of the two-stage charging mode, there is a problem in that the time-consuming small quantity charging is essential to ensure weighing accuracy and therefore the weighing speed cannot be increased beyond a certain extent.
The present invention resolves the foregoing problems, and an object of the present invention is to provide a weighing apparatus capable of weighing a powdery or granular material as a material to be weighed at high speed.